1. Field of the Invention
This invention relates to the field of pulsed magnetic electronic article surveillance (EAS) systems, and in particular, to generating accurate line synchronized delays for synchronizing operation of multiple EAS systems operating in proximity to one another.
2. Description of Related Art
When multiple pulsed magnetic EAS systems operate in proximity, their transmitter timing windows must never coincide with their receiver timing windows.
In certain pulsed magnetic EAS systems available from Sensormatic Corporation, the operation of such multiple EAS systems is synchronized by sensing the positive zero crossings of the local mains power line. Each line cycle is divided up into six time windows: three for transmission and three for reception, as shown in FIG. 3. The first transmit-receive window sequential pair, designated phase A, occurs at 0.degree. with respect to the zero crossing. The second transmit-receive window sequential pair, designated phase B, occurs at 120.degree. with respect to the zero crossing. The third transmit-receive window sequential pair, designated phase C, occurs at 240.degree. with respect to the zero crossing.
Ideally, two or more systems in proximity sense the same zero crossing and do not interfere with one another. Sometimes, however, the two or more EAS systems are connected to different phases of a building's three phase power supply, and it may be desired to delay one system's zero crossing signal, for example, from phase A to phase B. It can also sometimes happen through a building being mis-wired that two EAS systems can be connected with their respective zero crossings 180.degree. apart. This results in one system's transmitter burst being centered in the other system's receiver window and vice versa. Systems cannot function in this mode. When this happens, the usual approach is to rewire one system's input power leads. The rewiring process is tedious and can be dangerous.
In present Ultra*Max pulsed magnetic EAS systems available from Sensormatic Corporation, reliance has been placed on analog circuitry to produce long zero crossing synchronizing delays. For purposes of the inventive arrangements, long delay times refers to delays of up to one whole period of the power line frequency, namely 16.6 sec for 60 Hz power and 20 msec for 50 Hz power. In designs which did incorporate digital or microprocessor generated delays, these designs utilized digital delays only for fine adjustment of the analog delays.